1. Field of the Invention
The present invention relates to aluminum alloy products for use as finstock materials within brazed heat exchangers and more particularly to finstock materials having high strength and conductivity after brazing and good sag resistance. The invention also relates to a method of making such finstock materials.
2. Background Art
Aluminum alloys have been used in the production of automotive radiators for many years, such radiators typically comprising fins and tubes, the tubes containing cooling fluid. The fins and tubes are usually joined in a brazing operation. The finstock material is normally fabricated from a so-called 3XXX series aluminum alloy where the main alloying element added to the aluminum melt is manganese (see “International Alloy Designations and Chemical Composition Limits for Wrought Aluminum and Wrought Aluminum Alloys”, published by The Aluminum Association, revised in January 2001; the disclosure of which is specifically incorporated herein by this reference).
There is a continuous need for improved finstock materials to satisfy the demand for reductions in vehicle and component weight. In order to achieve weight reductions various properties need to be optimized. Principally, that means maintaining or improving the strength of the finstock material after brazing, without detriment to the thermal conductivity and the sag resistance. Sag resistance is resistance to high temperature creep during the brazing cycle which is the main reason for collapse of fins during the brazing of heat exchanger units. Thermal conductivity, of course, has a direct impact on the thermal performance of the heat exchanger unit, the other properties being essential for the structural stability of the unit. Besides these properties, the finstock must provide sacrificial protection to the tubes whilst avoiding deterioration through corrosion. It is common practice to make the fins electronegative relative to the tubes so that the fins act as sacrificial anodes. There is a need to balance this sacrificial effect with the need to maintain thermal performance during the service life of the heat exchanger. If the fins corrode too quickly thermal performance is compromised.
European Patent Publication EP1918394 describes a method of making an Al—Mn foil for use as fins in heat exchangers in which an alloy is used within the following composition range (all composition values hereinafter are expressed in weight %): 0.3-1.5 Si, ≦0.5 Fe, ≦0.3 Cu, 1.0-2.0 Mn, ≦0.5 Mg, ≦4.0 Zn, ≦0.3 of each of elements from group IVb, Vb or VIb elements, the sum of these elements being ≦0.5, unavoidable impurities and the remainder aluminum. The alloy may be twin roll cast, rolled, interannealed, cold rolled again, and then heat treated to avoid recrystallization of the foil. Although pre- and post-brazing strengths are reported, the electrical conductivity is not stated.
European Patent Publication EP1693475 describes an aluminum fin alloy with 1.4-1.8 Fe, 0.8-1.0 Si and 0.6-0.9 Mn where the surface grain structure is controlled such that more than 80% of the grains are recrystallized. This alloy was continuously cast by twin roll casting. Although sag resistance and electrical conductivity were good, the strength after brazing was below 140 MPa. The microstructure is characterised by the presence of Al—Fe—Mn—Si intermetallics.
European Patent Publication EP2048252 describes an aluminum fin alloy with the following composition: Si 0.7-1.4, Fe 0.5-1.4, Mn 0.7-1.4, Zn 0.5-2.5, other elements ≦0.05, balance aluminum where the sheet product has an Ultimate Tensile Strength (UTS) after brazing ≦130 Mpa and a Yield Strength (YS) ≧45 Mpa, a recrystallized grain size ≧500 μm and an electrical conductivity ≧47IACS. This product is manufactured from a belt cast strip, the thickness of the cast strip being between 5 and 10 mm.
US Patent Publication US-A-2005/0106410 describes a clad finstock material wherein the core material consists of an alloy containing 0.10-1.50 Si, 0.10-0.60 Fe, up to 1.00 Cu, 0.70-1.80 Mn, up to 0.40 Mg, 0.10-3.00 Zn, up to 0.30 Ti, up to 0.30 Zr, balance Al and impurities, and the clad layer is an Al—Si based alloy. No thermal conductivity data are reported. The post-braze strength reported was 136 or 146 MPa but the actual alloys which provided these values are not stated.
U.S. Pat. No. 6,620,265 describes twin roll casting an aluminum alloy with the following main alloying elements: 0.6-1.8 Mn, 1.2-2.0 Fe and 0.6-1.2 Si, where the casting load is controlled, and including at least two interannealing steps during cold rolling and in such a way as to avoid complete recrystallization. Sag resistance and conductivity were good but post-brazing strength was below 140 MPa.
US Patent Publication US-A-2005/0150642 describes an aluminum finstock material comprising the following composition: about 0.7-1.2 Si, 1.9-2.4 Fe, 0.6-1.0 Mn, up to about 0.5 Mg, up to about 2.5 Zn, up to about 0.10 Ti, up to about 0.03 In, remainder aluminum and impurities. This finstock material, which can be continuously cast, provides a conductivity >48% IACS and a post-brazing strength of >120 MPa. After a commercial brazing cycle involving a cooling rate of around 70° C./minute from the peak temperature to below 500° C., the post-braze strength was 130 or 131 MPa.
U.S. Pat. No. 7,018,722 describes a clad finstock material comprising a core and two clad layers, the core composition being selected from a wide range and the clad layers being selected from an Al—Si alloy. The invention concerns controlling the Si content in the core layer so that there is a difference between the Si concentration at the surface (0.8 or more) and in the middle of the core (0.7 or less). No mechanical property data or electrical conductivity data are reported.
PCT patent publication WO07/013,380 describes an aluminum alloy for use as finstock comprising the following composition: 0.8-1.4 Si, 0.15-0.7 Fe, 1.5-3.0 Mn, 0.5-2.5 Zn, remainder impurities and aluminum. This alloy is produced by twin belt casting. Although the strength levels after brazing are good, the conductivity is relatively low with a maximum reported value of 45.8% IACS.
U.S. Pat. No. 6,592,688 describes a continuously cast alloy containing 1.2-1.8 Fe, 0.7-0.95 Si, 0.3-0.5 Mn, 0.3-1.2 Zn, balance Al. The conductivity after brazing was >49.8% IACS and the post-brazing strength was >127 MPa. None of the examples showed a post-brazing strength above 140 MPa.
U.S. Pat. No. 6,165,291 describes a process for making finstock material where the process is applicable to alloys within the following compositional range: 1.2-2.4 Fe, 0.5-1.1 Si, 0.3-0.6 Mn, up to 1.0 Zn, other elements <0.05 and balance Al. The process involves twin roll casting to provide very high cooling rates during casting together with control of the cold rolling and interanneal conditions. The resulting finstock material is reported to have a conductivity greater than 49% IACS with a post-braze strength >127 MPa.
U.S. Pat. No. 6,238,497 describes a method of producing aluminum finstock material comprising continuously casting a strip, optionally hot rolling and then cold rolling, interannealing and further cold rolling. The method is applied to an alloy having the composition: 1.6-2.4 Fe, 0.7-1.1 Si, 0.3-0.6 Mn, 0.3-2.0 Zn, other elements <0.05 and balance Al. The resulting finstock material is reported to have a conductivity greater than 49% IACS with a post-braze strength >127 MPa.
The balance of properties varies from one reference to another. Occasionally a high thermal conductivity can be achieved but this is at the expense of strength after brazing. In other cases the situation is reversed.
It would be desirable to provide a finstock material having high strength and conductivity after brazing, with sufficient corrosion performance to ensure there is sacrificial protection to the tubes of the heat exchanger whilst avoiding rapid deterioration of the fins.